Hi there, you will not have a 'spine jarring' ride using 18's. I have owned my Gti for two years and it uses 18's (monza 2s). The ride is excellent. Yes it's firm compared to the standard golf but its not harsh or uncomfortable in any way. My car is on standard suspension.
Given, I've never driven a Gti on 17's. IMO that the 17" monzas don't look nearly as good as 18's. 19's look even better though ;)